[DuckDBPyType] Add __hash__ method overload

[Tishj]

This PR fixes #78

[Tishj]

I think we stumbled into another windows define..

    python_conversion.cpp
  D:\a\duckdb-python\duckdb-python\src\duckdb_py\include\duckdb_python\pytype.hpp(33,10): error C3646: 'Hash': unknown override specifier [C:\Users\RUNNER~1\AppData\Local\Temp\tmpfoywn8np\build\src\duckdb_py\native\python_native.vcxproj]
    (compiling source file 'D:/a/duckdb-python/duckdb-python/src/duckdb_py/native/python_conversion.cpp')